Calcifying cyanobacteria—the potential of biomineralization for carbon capture and storage
Introduction
Strategies to reduce emissions of carbon dioxide (CO2) from fossil fuels, and hence mitigate climate change, include energy savings, development of renewable biofuels, and carbon capture and storage (CCS). For CCS, several scenarios are being considered. One approach is capture of point-source CO2 from power plants or other industrial sources and subsequent injection of the concentrated CO2 underground or into the ocean [1]. An alternative to this point-source CCS method is expansion of biological carbon sequestration of atmospheric CO2 by measures such as reforestation, changes in land use practices, increased carbon allocation to underground biomass, production of biochar, and enhanced biomineralization [2]. In addition to geological or oceanic CO2 injection, novel models for point-source CCS based on accelerated weathering and biomineralization are emerging, utilizing either abiotic [3, 4, 5] or biotic [4, 6, 7] processes.
Biomineralization of CO2 by calcium carbonate (CaCO3) precipitation is a common phenomenon in marine, freshwater, and terrestrial ecosystems and is a fundamental process in the global carbon cycle [8].
Precipitation of CaCO3 can proceed by either or both the following reactions:Ca2+ + 2HCO3− ⇆ CaCO3 + CO2 + H2OCa2+ + CO32− ⇆ CaCO3with reaction (2) being the principal path, at least in seawater [9, 10].
Bicarbonate (HCO3−) is ubiquitous in water and is formed via dissolution of gaseous CO2:CO2(aq) + H2O ⇆ H2CO3H2CO3 ⇆ HCO3− + H+
The concentration of carbonic acid (H2CO3) is small so the dissolved CO2 from reactions (3) and (4) occurs predominantly as HCO3−.
A fraction of HCO3− dissociates to form carbonate (CO3−):HCO3− ⇆ H+ + CO32−
The lion's share of global calcification takes place through biotic processes in the oceans. Although the oceans are supersaturated with Ca2+ and CO3−, spontaneous precipitation of CaCO3 in the absence of calcifying (micro)organisms is rare owing to various kinetic barriers [11]. The contribution of microorganisms, particularly cyanobacteria, in CaCO3 precipitation and sedimentation is substantial and it has played a major role in geological formations since the Archaean Era [12]. Although studies of microbially mediated biomineralization through CaCO3 precipitation have a long history, the mechanistic details of the different steps are only poorly understood [13•]. In this review we discuss the potential for microorganisms, specifically cyanobacteria, in calcification, that is conversion of CO2 to recalcitrant calcium CaCO3.
We begin our discussion on cyanobacterial calcification and its potential in CCS by a brief description of the general features of cyanobacteria where we elaborate on the carbon-concentrating mechanism (CCM) that allows cyanobacteria to actively take up inorganic carbon (Ci) from the external medium and perform efficient photosynthesis in aqueous environments. We then give an account on microbial biomineralization, specifically as it occurs in cyanobacteria. In this context we return to the CCM and point out the intimate association between CCM and the calcification process. Finally, we ask how biomineralization by calcifying cyanobacteria can contribute to CCS, and we point out research areas that should be prioritized to tackle some of the challenges ahead.
Section snippets
Cyanobacteria
Cyanobacteria are photosynthetic Gram-negative bacteria that carry out oxygenic photosynthesis and are thought to be the origin of chloroplasts of plants and eukaryotic algae via endosymbiotic events in the late Proterozoic or early Cambrian period. Cyanobacteria occupy a wide array of terrestrial, marine, and freshwater habitats, including extreme environments such as hot springs, deserts, bare rocks, and permafrost zones. In their natural environments, some cyanobacteria are often exposed to
Biomineralization by calcifying cyanobacteria
The occurrence and distribution of calcifying microorganisms are widespread [37, 38, 39]. A number of microbial strains capable of calcification have been reported, for example various cyanobacteria, eukaryotic microalgae, Bacillus, Pseudomonas, Vibrio, and sulfate-reducing bacteria. Although the phenomenon of microbial calcification has long been recognized, its physiological function is unknown. It might confer a selective advantage in providing a protective shield against high-light exposure
CCS using calcifying cyanobacteria
Through photosynthesis and calcification, cyanobacteria have the potential to capture CO2 from flue gas and store it as precipitated CaCO3. Calcium is abundant in many terrestrial, marine and lacustrine ecosystems. By using halophilic cyanobacteria, seawater or brines, for example agricultural drainage water, or saline water produced from petroleum production or geological CO2 injections, can serve as potential calcium sources for the calcification process. Calcification can further be boosted
Conclusions
Employment of cyanobacteria for point-source CCS of flue gas via calcification offers promising strategies for reducing anthropogenic CO2 emissions. However, much research is urgently needed to further our understanding of the biochemical and physical processes in cyanobacteria that promote calcification, and that will allow us to select or design strains with optimized properties for specific applications and conditions using genetic engineering or directed evolution. For example, it is
References and recommended reading
Papers of particular interest, published within the annual period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
This work was supported in part by U.S. Department of Energy Contract DE-AC02-05CH11231 with Lawrence Berkeley National Laboratory.
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